In recent years, the volume of the Japanese short neck clam Ruditapes philippinarum imported from China, South Korea and North Korea has increased rapidly to compensate for the decrease in domestic production in Japan. Imported clams are generally released in areas already inhabited by the same species, and it is feared that they may affect the domestic populations. It is necessary to understand the genetic characteristic of the various local populations to manage and conserve biodiversity. However, the genetic variation of the Japanese short neck clam from Japan and the other countries has not been studied. This study was therefore designed to analyze the genetic variation among local populations from Japan and China using nucleotide sequence of the M and F types COX1 gene from the mtDNA of Japanese short neck clam. The nucleotide sequence for the total length of the M-type (1608 bp) and F-type (1599 bp) COX1 gene of 39 male individuals collected during the spawning season (November to May, 2004) from nine locations in Japan and China was determined. In the M-type COX1 gene, the number of nucleotide and amino acid substitutions were higher than in the F-type COX1 gene. Both the M-type and F-type COX1 gene revealed differences in nucleotide sequence between Japanese and Chinese populations, which were clearly grouped in the phylogenetic trees (MP and NJ methods). In addition, the Japanese populations were divided into two subgroups by the M-type COX1 gene, the Honshu-Kyushu (Tokyo Bay, Mikawa Bay, Nanao Bay, Miyazu Bay and Ariake Sea) and Hokkaido (Notsuke Bay) groups. The Chinese populations were also divided into two subgroups, these from the North (Dalian Bay and Kiachow Bay) and those from the South (Xiamen Bay).
Two genetic types (A and B) of Anodonta "woodiana" were identified as A. lauta and A. japonica respectively, based on the number of large teeth on the hook of glochidia, which was always more than 12 in A. lauta, but less than 11 in A. japonica on the average. The ratio of shell height to shell length in the glochidia was not a suitable character to identify these two species, because it varied largely in A. japonica.
To reveal the mechanism of androgenesis, we observed chromosomes, centrosomes and microtubules in fertilized C. fluminea eggs from meiosis to first mitosis. We also observed fertilized eggs of C. sandai, in which development is normal. In C. sandai, one of the centrosomes attached to the egg cortex and the other remained in the center of eggs at metaphase of meiosis. The spindle axis at metaphase of meiosis was perpendicular to the egg cortex. On the other hand, in androgenetic eggs of C. fluminea, two centrosomes attached to the egg cortex. The spindle axis was parallel to the egg cortex. As a result, all egg chromosomes were extruded with two polar bodies at first meiosis. Only the male pronucleus remained in eggs after the polar body formation. C. fluminea has two centrosome attachment sites, while C. sandai has only one attachment site at meiosis. We deduced that the change of attachment site may cause the androgenesis in C. fluminea. We also measured the size of male pronucleus in C. sandai and C. fluminea fertilized eggs during meiosis. At 20 min after fertilization, all egg chromosomes were extruded with two polar bodies in C. fluminea. Meiosis of C. fluminea was apparently completed at 20 min, but the male pronucleus didn't enlarge until 40 min had elapsed. At 40 min, the male pronucleus started to enlarge, which coincides with the period of second polar body formation in C. sandai. We suggest that the cell cycle of second meiosis is still functional in oocytes of C. fluminea.